1. Field of the Invention
The present invention relates to a composite material substrate for solar cells comprising semiconductor materials, and to a technique of producing large-sized solar cells each having a large area panel.
2. Discussion of the Background
Devices with thin semiconductor films require substrates for the thin films. Semiconductor films are formed on substrates through chemical vapor deposition (CVD) or thermal spraying. In general, the films as formed in such methods are not crystalline enough to satisfactorily exhibit their semiconductive properties, and therefore must be subjected to an additional melting and recrystallizing step to produce monocrystalline or polycrystalline morphology. The resulting monocrystalline or polycrystalline films of Si or the like could be photoelectric transfer layers for solar cells. Therefore, the substrates for semiconductor films must be resistant to heating at the melting point of semiconductor materials, which is extremely high and is often above 1000.degree. C. In addition, the thermal expansion coefficient of the substrates must conform with the thermal expansion coefficient of semiconductor materials. This is because the semiconductor layers to be formed on the substrates must not crack at high temperatures at which they are formed and also at room temperature, without being influenced by the thermal expansion of the underlying substrates. Heretofore, heat-resistant materials of aluminium oxide, magnesium oxide, silicon oxide and the like having a low thermal expansion coefficient have been used for the substrates. However, the substrates of those materials are still problematic when their size is large. For example, when they have a size of 3 inches or larger, the semiconductor films formed thereon are often cracked.
In particular, for large-sized solar cells in which the panel has a large area of at least 10 inches in diameter, the thermal conformity as above between the substrates and the semiconductor films to be formed thereon is an important factor and is indispensable. Conventional substrates do not conform well to semiconductor materials such as silicon, and often induce cracks in Si layers or even crystal defects therein, which are fatal to devices. For popularizing solar cells as power-generating systems installed on roofs for household use, their structure must have sufficiently high strength to resist typhoons, hurricanes, strong winds, and other rough weather. However, substrates made of available materials as above which are large in size but not thick do not have high strength, and are often broken. Conventional solar cells have a unit panel size of at most 100 cm.sup.2, and, at present, are not large-sized solar cell panels that satisfy the requirements noted above. It is desirable to have a technique to enlarge the size of solar cells by enlarging the area of a unit cell or panel in solar power devices, and not merely integrate a plurality of solar cells into one solar power device.